Manager Mike Hargrove said Pineiro would spend 10 days working on his mechanics with Tacoma pitching coach Rafael Chaves, and the team plans to bring him back May 24 to pitch against Baltimore.
"I think this gives Joel the best chance to get back being consistent with his delivery," Hargrove said. "This is not a health issue."
The Mariners will also push back Jamie Moyer's next start. Moyer will pitch on Wednesday against the Yankees, with reliever Julio Mateo moving into the rotation and pitching on Tuesday. Hargrove said the move was made to split Aaron Sele and Moyer in the rotation.
Pineiro, who started the season on the disabled list with a strained right elbow, is 2-3 with a 6.52 ERA in six starts and has not won since April 26 at Texas. He's given up six runs in each of his last two starts, and Friday was his shortest outing since lasting just 3 2/3 innings against Boston last July.
Pineiro did not comment before the game, telling reporters to talk to Seattle's coaches.
"I'm sure he wasn't real happy," Hargrove said. "But he wasn't kicking and screaming."
There are no plans at this time for Pineiro to start any games in Tacoma. Pitching coach Bryan Price said Pineiro acknowledged something was wrong in his delivery but said the problem is very subtle.
"The most important thing is that when he is on the mound, he is comfortable and confident," Price said. "Being confident goes with being comfortable with your mechanics and having the expectation that you're going to make good pitches."
Hargrove said the team would probably recall left-hander George Sherrill to help Seattle's bullpen.
Mateo started just seven games in the minors. He got the win in relief of Pineiro on Friday and has allowed just one earned run in 22 innings pitched this season.
"He's happy for the chance," Hargrove said.